Semi-sealed silencer structure

ABSTRACT

A drum which receives explosion impulse waves directly on the inner surface thereof is made of steel, and a material inlet tunnel and pipes for alleviating the explosion impulse and reducing the explosion noise are connected to said drum, and further the resultant structure is covered exteriorly with soil or concrete, whereby a semi-sealed silencer structure is provided which has substantially improved pressure-resistance and in which several tens to several hundreds kilograms of explosive can be exploded without causing dangers and public nuisances.

Yamamoto SEMI-SEALED SILENCER STRUCTURE [75 Inventor: Kazumoto Yamamoto, Saga-ken,

, Japan [73] Assignee: AsahiKasei Kogyo Kabushki Kaisha, Osaka, Japan 22 Filed: Oct. 2, 1972 21 Appl. No.: 294,159

[52] US. Cl 181/33 R, 181/33 G, 181/61, 73/35, 73/12, 86/20 C [51] Int. Cl. F01n 1/02' [58] Field of Search 181/33 K, 33 G, .5 R; 220/9 A, 9 R; 73/35, l2;'52/20 [56'] References Cited UNITED STATES PATENTS 7 2,028,968 1/1936 Carlstrom...'... 52/20 X 2,382,171 8/1945 Pomykala 52/20 X 3,165,916 l/l965 Loving, .lr 73/35 3,263,378 8/1966 Dorris 52/20 3,295,327 1/1967 Waterman 220/9 R [11] 3,823,793 [4 July 16,1974

Mueller 73/35 3,301,041 l/l967 3,318,144 5/1967 Duff 73/35 3,397,756 8/1968 Andrews et a1. 181/33 K Primary Examiner-Richard B. Wilkinson Assistant Examiner-Nit W. Miska Attorney, Agent, or Firm-Armstrong, Nikaido and Wegner 57 1 ABSTRACT A drum which receives explosion impulse waves di- 2 Claims, 7 Drawing Figures PATENTEU JUL 1 61974 SHEET 1 BF 3 SHEET 2 OF 3 PATENTEDJULI 61974 3' e23 793 SHEET 3 [IF 3 1 SEMI-SEALED SILENCER STRUCTURE This invention relates to a semi-sealed silencer structure adapted for use in testings and explosive workings utilizing a large amount of explosive.

In a testing or explosive working, inter alia explosion bonding or explosion hardening, utilizing a large amount of explosive, an explosive is occasionally used at a time in an amount as large as several hundreds of kilograms, and the explosion of such a large amount of explosive involves such problems that buildings in the neighbourhood of the spot of explosion are damagedby the explosion impulse waves and the noise generated by the detonation causes a public nuisance. I

For this reason, in the past such a work has been carried out at a place remote from residences or attempt has been made to alleviate the explosion impulse wave and noise by covering the structure with soil, in which the explosion takes place. However, the selection of the place where the explosive working 'or the like is to be carried out is subjected to limitation when the transportation of the metallic materials to be worked isconsidered because, in such a working, the weight of the metallic material, e.g. steel material, reaches as heavy as several tens to several hundreds tons. Further, the impulse and noise alleviating method using a covering ma terial has an insufficient practical effect when the amount of explosive used is as large as several tens of kilograms or more.

Concerning the reduction of impulse waves and noisesgenerated at the detonation of explosive, a few proposals have been made heretofore. For instance, Japanese Pat. No. 544,285 teaches the use of a semisealed noise-proof tank and US. Pat. No. 3,397,956 proposes a method in which explosion is carried out in a tunnel which is sealed with a movable plug and in which water is sprayed. However, the former has a number of practical defects when the amount of explosive used is more than several tens of kilograms, because excessively high pressure acts on a closure cover ofthe tank, although it may be satisfactory for a partial explosion bonding or the like working, utilizingless than kilograms of explosive. In case of the latter in which explosion is carried out in a tunnel, while the tunnel may be designed to be of a structure capable of exploding a large amount of explosive therein, the materialization of a tunnel which satisfies all and any requirements is difficult since the strength of the tunnel is restricted by the selection of the location, and the kind and quality of the rock at the selected location, and in addition, the'construction cost tends to become extremely high.

For a working, such as explosive working, which has production for its primary object, it is an important factor to perform the working efficiently. To this end, it is ideal to perform the working at a location near the place of consumption of the materials produced by the explosive working, and the use of a silencer structure is preferable as a facility to meet such desire.

An additional advantage of performing the explosive working in such a structure is that the working efficiency is not influenced by weather conditions as is in case of working in the open air. However, the structure sed m s 299f a was. hat ns sgn dwblxa ssa space or large strength against pressure, by reason of the fact that the pressure created by detonation is unimaginary large and, for instance, when about 500 kilograms of explosive is detonated, is 200-400 kg/cm at a point 4 meters away from the source of detonation, though it is only 3O-70kg/cm at a point 8 meters away from the source of detonation. When the structure is of a substantially sealed type, the degree of damage to and displacement of the structure caused by detonation of an explosive therein are proportional to the impulse (the value of pressure integrated by time), as well as the maximum pressure, acting on the inner wall of the structure. This impulse is largest where the structure is completely sealed, but becomes appreciably small in case of a so-called semi-sealed structure which has some openings formed through the wall thereof.

It will be understood, therefore, that in the economic production of the silencer structure, a semi-sealed structure is preferable from the viewpoint of durability and, on the other hand, the area of the openings should be as small as possible in consideration of the silencing effect. Further, for the same total area of the openings, it is preferable, from the viewpoint of silencing effect, to disperse openings of a small area, rather than concentrating the openings into one location. This is advantageous also from the viewpoint of alleviating the impulse. In the arrangement as described above, the silencing effect becomes large because, in the absorption of noise by a pipe, the acoustic absorption efficiency of the pipe increases as the pipe diameter becomes smaller. With r representing the radius of the pipe and f representing the frequency of sound wave, the acoustic absorption efficiency [3 is expressed by ,8 0.65 V77r in the range of r 0.40 VTFurthermore, the impulse is alleviated because, when detonation is carried out in the structure, the explosion pressure acts on the structure wall not uniformly and with a time lag, and the provision of the openings in a dispersed fashion avoids the occurrence of abnormal stresses in the structure.

It is also effective to reduce the explosion noise by providing silencer means at each opening (as described, for example, on page 78 of the Journal of the Industrial Explosive Association, 1968, Vol. 29, published in Japan). In this method, however, the silencer means need be of a sufficiently rigid and simple structure, by reason of the fact that the velocity of gas blowing out through the opening reaches as high as 2,000 m/sec as the amount of explosive used is large. A pipe is preferably used as the most simple and effective silencer means. The effect of sound absorption by a pipe generally increases as the frequency of the sound becomes higher and the pipe diameter becomes smaller, but the length of the pipe is also important and should be at least 1.0 time the sound wave length. The frequency of the explosion sound in the vicinity of a source of explosion is -400 C/sec. For silencing the noise, therefore, it is considered that the length of the pipe need be at least 1,000 mm.

Such a structure as described above is a sort of explosion-proof structure, so that the strength against pressure of the structure generally increases with the weight thereof. When the whole structure is to be made of a steel material for increasing the weight thereof, the thickness of the steel plate used must be extremely large, which makes the production cost extremely high. Therefore, it is preferable to cover the steel structure exteriorly with a concrete or soil layer, so as to apply the pressure of said concrete or soil layer to the structure and thereby to substantially increase the weight and pressure-resistance of the structure. This method has the additional advantage that the fatigue of the steel material is greatly decreased since the vibration of the structure wall will attenuate very quickly. The concrete or soil layer also effectively reduces the so-called secondary noise which emanates into the atmosphere through the steel material.

From the weight point of view, it can be said that a reinforced concrete or PC concrete structure is also effective, but they cannot be used as a permanent structure durable with repeated detonation, since the separation of the concrete surface layer tends to be caused by the primary impulse waves at the time of detonation. Therefore, for the prevention of the concrete separation also, it will be advantageous to use a steel plate over the inner surface of the structure which directly receives the explosion impulse waves. In case of a semisealed silencer structure, the use of a steel material as its inner layer is further advantageous in that the structure can be designedwith a strength greater than the nominal design strength of the steel material and hence much economically, because a steel material generally exhibits a strength considerably greater than its nominal strength, against pressure acting thereon in a short period of time, such as explosion impulse.

The present invention has for its object the provision of a semi-sealed silencer structure which, when used in explosive workings or testings using large amounts of explosive, is capable of preventing dangers caused by the impulse waves or flying substances produced at the time of detonation of the explosive, of eliminating the public nuisance caused by the explosion noises and of enhancing the working efficiency.

According to the invention there is provided a semisealed silencer structure comprising a hollow drum made of a steel material, a material inlet tunnel and at least one pipe connected to said drum, said pipe being at least 1,000 mm in length and serving as a silencer, and a soil or concrete layer exteriorly covering said drum. 1

In the semi-sealed silencer structure according to the invention, a drum which directly receives explosion impulse waves on the inner surface thereof is made of a steel material, and a material inlet tunnel and at least one pipe, which alleviates the impulse and serves as a silencer, are connected to said drum, and further the structure thus obtained is covered exteriorly with a soil or concrete layer, whereby the strength of the structure is substantially increased to such a level that several tens to several hundreds kilograms of explosive may be detonated in said structure, with no dangers and without causing a public nuisance otherwise caused by the noise generated by the detonation.

In comparison with conventionally proposed silencer facilities, the silencer structure of the invention has the advantages that its production cost is substantially low, and that, when a routine productive working, such as explosive working, is performed therein, the working efficiency can be greatly enhanced.

The invention may be performed in various ways and a number of embodiments will now be described by way of example, and with reference to the accompanying drawings, in which:

FIGS. 1, 2 and 3 exemplifies a structure for explaining the present invention, which was used in the experiment conducted by the present inventor, and of these Figures,

FIG. 1 is a longitudinal sectional side view of a sealed structure made of a steel material;

FIG. 2 is a side elevational view of the structure of FIG. I, with silencer pipes connected thereto; and

FIG. 3 is a transverse sectional front view of the silencer structure of FIG. I, with a concrete cover layer, in which the structure of FIG. 2 is shown in section taken along the line IIIIII of FIG. 2;

FIG. 4 is a longitudinal sectional side view showing an internal steel structure portion only of an embodiment of the semi-sealed silencer structure according to the invention;

FIG. 5 is a transverse sectional front view of the complete semi-sealed silencer structure, with the steel structure portion of FIG. 4 embedded in the ground,

said steel structure portion being in section taken along the line VV of FIG. 4;

FIG. 6 is a longitudinal sectional side view of another embodiment of the semi-sealed silencer structure of the invention in its completed state; and

FIG. 7 is a transverse sectional front view of the structure taken along the line VII-VII of FIG. 6.

A few experimental examples of the present invention will be described hereunder: With reference to the vertical sectional side view of FIG. 1, the sealed steel structure shown is composed of a cylindrical drum 1 and end plates 2, 2. The cylindrical drum 1 has a wall thickness of 18 mm, an inner diameter of 700 mm and a length of 1,200 mm. The end plates 2, 2' each has a thickness of 23 mm and a sectional shape which is the half of an ellipse having the major axis-to-minor axis ratio of 2 1. The end plate 2 is welded to the cylindrical drum 1, while the end plate 2 is secured to the cylindrical drum 1 by means of bolts 3. All of the cylindrical drum 1 and the end plates 2, 2' are made from a SB42B steel sheet specified in the Japanese Industrial Standards.

Inside the structure constructed as described above is placed sand 4 in a manner to form a sand layer having the maximum depth of 170 mm. A stratiform explosive 6 is set on a steel plate 5 placed on the sand layer 4, the thickness of said steel plate 5 being 25 mm. An arrangement is made such that the stratiform explosive is detonated by an electric detonator 7.

FIG. 2 is a side view of the steel structure shown in FIG. 1, with silencer pipes 8 connected to the cylindrical drum thereof. These silencer pipes serve to alleviate the explosive impulse and reducing the noise generated by detonation and are provided at two locations on the top of the cylindrical drum 1 and at three locations on each side of said cylindrical drum at a level flush with the axis of said cylindrical drum. Three types of pipes having a diameter of mm and lengths of 700, 1,200 and 2,000 mm respectively are selectively used and are exchangeably connected to the cylindrical drum 1 at flanges thereof. Further, strain gauges S S and S are attached to the structure wall at locations indicated by the A markings respectively to measure the stresses occurring in the wall of said structure.

FIG. 3 is a transverse sectional front view of the structure shown in FIG. 2, which is covered exteriorly with concrete, the length of the pipes being 2,000 mm. In FIG. 3, reference numeral 9 designates a reinforced concrete which is provided around the structure to enclose the same, the minimum thickness of said concrete being 800 mm.

An explosive was detonated in each of the sealed structure (a) shown in FIG. 1, the semi-sealed structure (b) including the 2,000 mm long pipes, shown in FIG. 2, as mounted on the ground, and the semi-sealed structure including the 2,000 mm long pipes and covered with the reinforced concrete, shown in FIG. 3, and the stresses occurring in the walls of the respective structures were measured, with the results shown in Table I. In the experiment, the strain gauges S S and S were attached to the structures (a) and (b) at locations corresponding to the locations shown in FIG. 2.

The explosive used was one comprising PETN and ammonium nitrate as its main components, and the velocity of detonation was about 3,000 m/sec.

As may be understood from the above table, the stresses occurring in the structure walls are much smaller in case of the semi-sealed structures (b) and (c) than in case of the sealed structure (a); It will also be understood that, while the structure (0) according to the invention shows only a slight reduction in maximum stress as compared with the structure (12), it reduces remarkably the frequency of. the explosion noise and to about 1/10 of that in case of the structure (a) and to about /a of that in case of the structure (b), which is of great advantage in respect of fatigue of the steel material of which the respective structures are made. No substantial differences were noted with respect to stress and frequency between when the pipe length was 2,000 mm and when the pipe length was 700 mm or 1,200 mm. Further, as will be apparent-from Table l, the steel structure was not subjected to permanent deformation despite of the fact that a stressof 60 kg/mm or larger, which exceeds the static tensile breaking strength of the steel material, acted in the structure wall. This means nothing but that the steel material shows a higher value than its nominal strength value, against dynamic impulse.

Then, the explosion noises were measured at a point 100 meters away from the place where each structure was located, by means of an indication noise meter. The results are shown in Table 2. In Table 2, the column heading Open air" means that the explosive was detonated on the ground in the open air; Semi-sealed (without pipes, on the ground) means that the explosive was detonated in the cylindrical drum 1 only of the structure of FIG. 2, which had been placed on the ground upon disconnecting the silencer pipes 8 therefrom and hence been in such a state that the interior of said drum communicates with the atmosphere through the plurality of the openings formed through the wall thereof; and Semi-sealed (pipe length 700) and Semi-sealed (pipe length 1,200) respectively mean that the explosive was detonated in the structure of FIG. 2 having the silencer pipes of a length of 700 mm and in the same structure but having the silencer pipes of a length of 1,200 mm.

TABLE 2 As may be apparent from Table 2, the noise level of the detonation carried out in the open air is quite high as compared with that of detonation carried out in the structures. It will also be understood that, under the experimental conditions described above, when the silencer pipe length is 700 mm, the silencer pipes lower the noise level only 2-3 db from the case when the detonation was carried out without pipes, but when the silencer pipe diameter is 1,200 mm or larger, the noise level is lowered almost 10 db, which signifies that the silencer pipes have a considerable noise reducing effect. Further, when the pipe length is 700 mm, the lowering of noise level is only 2-3 db from the case of semi-sealed (without pipes) In the case of (c) wherein the steel steel structure is covered with concrete, the noise reducing effect is slight, i.e., the lowering of the noise level is only l2 db, which means that the secondary noise is not a substantial problem. The fact that the pipe length has a large influence on the noise reducing effect, as stated above, will be understood also from the sound wave length in the vicinity of the source of detonation which is assumed to be about 1-4 mm. Thus, in view of the sound absorption effect, it is preferable that the silencer pipe length is longer than the sound wave length.

Now, examples of the present invention will be described practically hereunder with reference to FIGS. 4-7.

' EXAMPLE 1.

In FIGS. 4 and 5, reference numeral 1 designates a cylindrical steel drum made from a 12 mm thick SS41 steel sheet specified in the Japanese Industrial Standards. Clamp belts 11 each made from a 200 mm long equal-angle steel are welded to the outer surface of the cylindrical drum 1 at five locations at an interval of 2,000 mm. The inner diameter of the cylindrical drum 1 is 6,000 mm and the linear axial length thereof is 8,000 mm. Reference numerals 2, 2' respectively designate end plates made from a 12 mm thick SS41 steel sheet and welded to the opposite ends of the cylindrical drum l. Reinforcement members 12 are similarly welded to the outer surfaces of the end plates 2, 2,

each of which consists of a 200 mm long equal-angle steel.

A tunnel 14 for carrying a material into and out of the cylindrical drum 1 therethrough has a generally semicylindrical shape having a width of 2,400 mm, a height of 1,400 mm and a length of 6,000 mm. This tunnel 14 is similarly made from a 12 mm thick SS4l steel and welded to the central portion of one side of the cylindrical steel drum 1.

Pipes 8 for reducing explosion impulse and noise are connected to the top of the cylindrical drum 1 as shown in FIG. 4. Each pipe 8 is made from a $541 steel sheet having a thickness of 9.5 mm and has a length of 4,500

4 as embedded in the ground, and as seen, sand 15 is tightly disposed around the cylindrical drum 1, the end plates 2, 2' and the tunnel 14. The sand 15 is further surrounded by soil 16. Reference numeral 17 designates a breast wall serving as sand guard. The total thickness of the sand 15 and soil 16 above the cylindrical drum 1 is about 4,000 mm.

An explosion bonding was carried out in the semisealed silencer structure, constructed as described above, using 36-42 kg of an explosive, and the stresses occurring in the cylindrical drum 1 and the angle steels 2 were measured. The maximum stress was 40-50 kg/mm in the cylindrical drum and 14-20 kg/mm in the angle steels, and neither breakage nor permanent deformation of these members was observed.

The noise generated by the detonation at this time was measured and compared with the noise generated when the same amount of explosive was detonated in the open air, and it was revealed that, at a point 400 meters distant from the point of detonation, the former was 82-94 db while the latter was 104-116 db, which means that a noise reduction of about db is possible.

EXAMPLE 2 FIGS. 6 and 7 show another form of the semi-sealed silencer structure which is of a larger size than that described in Example 1. In this form, a cylindrical steel drum 1 is made from a 12 mm thick SB42B steel sheet, and has an inner diameter of 9,600 mm and'a linear length of 10,000 mm.

End plates 2, 2' are each made from a 16 mm thick SB42B steel plate and welded to the opposite ends of the cylindrical steel drum 1, the radius of curvature of said end plate being 7,500 mm.

The cylindrical steel drum 1 is reinforced by reinforcement members 11, each consisting of a 250 mm l-steel, which are welded to the outer surface of said cylindrical drum, in the circumferential direction at an interval of 1,000 mm and in the axial direction at an interval dividing the outer surface equally into 12 sections. Each of the end plates 2, 2 is reinforced by 12 reinforcement members 12, each consisting of a 250 mm l-steel, which are welded to the outer surface of said end plate diametrically for connection with the axial reinforcement members 11 of the cylindrical drum 1 respectively.

A tunnel 14 for carrying a material into and out of the cylindrical drum therethrough is made from a 12 a 8 mm thick SB42B steel sheet, and has a width of 3,000 mm, a height of 2,200 mm and a length of 10,000 mm. This tunnel 14 is reinforced by reinforcement members 10, each consisting of a 150 mm I-steel, which are welded to the outer surface of said tunnel 14 in both the circumferential and axial directions.

The cylindrical drum 1 is formed with four openings at the top, and five openings at the center of each side thereof, and noise reducing pipes 8 are connected to said openings, extending vertically and horizontally respectively, as shown. Each of the pipes 8 is a steel pipe having a diameter of 350 mm and a thickness of 9 mm, and the length of the vertical pipe is 6,000 mm and that of the horizontal pipe is 11,000 mm. Reference numeral 13 designates a support for the pipe 8.

The outer surfaces of the cylindrical drum and end plates are covered with a reinforced concrete layer 9 having a thickness of about 1,000 mm, which is further covered with a soil layer 16. The thickness of the soil layer 16 above the cylindrical drum 1 is about 4,500 mm. The reinforced concrete layer 9 has embedded therein 22 mm diameter deformed bars arranged at an average interval of 150 mm.

Reference numeral 4 designates sand disposed in the steel structure composed of the cylindrical drum 1 and end plates 2, 2'; 17 a breastwall provided at the outer end of the tunnel 14 add serving as sand guard; 18 in FIG. 7 a concrete base fixedly supporting the horizontal pipes 8; and 19 a breast wall provided at the outer ends of the horizontal pipes 8 on each side of the cylindrical drum and serving as sand guard.

An explosion bonding was performed in the semisealed silencer structure thus constructed, using -140 kg of an explosive, and the noise generated by the detonation was measured at a point 500 meters away from the structure and found to be 84-96 db, while the noise measured at the same distance when the same amount of explosive was detonated in the open air was 118-130 db. Namely, it was found that the silencer structure is capable of reducing the noise about 30 db.

Thus, by the use of the semi-sealed silencer structure according to this invention, it has become possible to eliminate the public nuisance caused by explosion noises, not speaking of damages to facilities in the neighbourhood of the cite of detonation, and also to carry out explosive workings with high efficiencies. In the experiment described above, the semi-sealed silencer structure was not subjected to any destruction or permanent deformation, and thus proved that it can well be put in practical use, also from the strength point of view, for detonation of said amount of explosive.

As stated above, the semi-sealed silencer structure of the type composed of an internal structure made of steel, and a soil layer externally covering said internal steel structure exerting its pressure directly thereon or a reinforced concrete layer externally covering said internal steel structure or said reinforced concrete layer and a soil layer externally covering said reinforced concrete layer, in which said internal steel structure has connected thereto in communication therewith a material inlet tunnel and pipes of 1,000 mm or longer to reduce the impulse and noise generated by detonation performed in said internal steel structure, is effectively used in such workings as explosion bonding utilizing large amounts of explosive, for alleviating the impulse wave and hence preventing damages to nearby facilities 9 and for reducing the explosion noise which otherwise would be a public nuisance, and further substantially enhances the working efficiency.

The shape and space volume of the silencer structure, the material and size of the internal steel structure, The thicknesses of the soil layer and reinforced concrete layer, etc. are determined mainly by the amount of explosive used and the type of working, and the size of the tunnel is determined by the size of the material to be carried into the structure.

The size of the noise reducing pipes is determined, in a strict sense, in relation with the amount of explosive used, the space volume of the internal steel structure, etc., but should be at least 1,000 mm in length, because the frequencies of explosion noises in the vicinity of the source of detonation are, in most cases, assumed to be 120-400 C/sec. The shape of the pipes cannot be specified but it is preferable to determine the shape such that the sum of the cross sectional areas of all pipes and the tunnel, that is the total area of the openings, will be not larger than 25 percent of the longitudinal or transverse cross sectional area of the internal steel structure, whichever smaller.

The internal diameter of the individual pipe is determined by the number and length of the pipes used, but steel pipes of a diameter of 75-l ,200 mm are generally used.

When the size of the tunnel makes the aforesaid total area of openings-to-the longitudinal or transverse cross sectional area of the internal steel structure ratio to be 25 percent or larger due to the size of the material to be carried through said tunnel, or when a greater noisereducing effect is desired even if said ratio is not larger than 25 percent, it is possible to provide known silencer means (eg the one described on page 78 of Journal of The Industrial Explosive Association, Vol. 29, published in Japan in 1968 or Japanese Pat. No. 544,285) at all openings including the opening of the tunnel, and further it is possible to provide a breast wall in front of each opening to deflect and disperse the impulse wave or gases produced by detonation.

For carrying a material into the silencer structure of the invention, a known portable crane or stationary expansion crane may be used.

It is to be understood that sand bags, lumbers or steel sheets may be provided over the inner surface of the steel structure to protect the same against damage possibly caused by flying substances, such as fragments of steel, formed by detonation. Means and a method of such protection are not specified in the present invention as they are not part of the invention.

What is claimed is:

l. A semi-sealed silencer structure comprising a ho]- low steel drum, horizontal material inlet tunnel means connected to said drum for providing access to said drum to place material therein, horizontal and vertical pipe means coupled to said drum for reducing noise caused by an explosion in said drum, and concrete layer means covering the exterior of said drum.

2. The structure of claim 1 further including a soil layer covering said concrete layer means. 

1. A semi-sealed silencer structure comprising a hollow steel drum, horizontal material inlet tunnel means connected to said drum for providing access to said drum to place material therein, horizontal and vertical pipe means coupled to said drum for reducing noise caused by an explosion in said drum, and concrete layer means covering the exterior of said drum.
 2. The structure of claim 1 further including a soil layer covering said concrete layer means. 